1. Field of the Invention
The present invention relates to magnetic resonance imaging for imaging the inside of a subject (patient) on the basis of a magnetic resonance phenomenon occurring in the subject. More particularly, this invention is concerned with a magnetic resonance imaging (MRI) system and magnetic resonance (MR) imaging method which utilize an ECG gating technique employed as a cardiac synchronization technique.
2. Description of the Related Art
Magnetic resonance imaging (MRI) is a technique for magnetically exciting nuclear spins in a subject positioned in a static magnetic field by applying a radio-frequency (RF) signal with the Larmor frequency, and reconstructing an image using an MR signal induced with the excitation.
For imaging the vessels in the lungs or the vessels in the liver (portal vein) according to the magnetic resonance imaging, various requirements must be satisfied. One requirement is to improve a signal-to-noise ratio by raising signal levels representing a vascular image. Another one is to minimize artifacts caused by body motions.
Under those requirements, various types of magnetic resonance imaging methods are used. One method is an echo planar imaging (EPI) method known as ultra-fast imaging, which uses a pulse sequence in which MR signals are acquired by fast reversing the polarities of a read-out gradient after one RF excitation. The EPI imaging has the advantages of shorter data acquisition times and less artifacts caused by the body motion. Another one is imaging based on either a fast SE (spin echo) method or an imaging technique to which the fast SE method is applied, which is a relatively longer acquisition time per one time of RF excitation (i.e., one shot) compared to one heart beat (for example, approx. 300 msec, particularly 600 msec for larger matrix sizes). Imaging based on the fast SE method, which is longer in the acquisition time than an EPI (echo planar imaging) method, has the advantage of higher resistance to susceptibility and less distortions in shapes. Thus, imaging making use of those advantages can be made. An ECG gating technique employed as one of the known cardiac synchronization techniques can be used in those types imaging.
On the other hand, where an object to be imaged is the heart system, imaging can also be performed with FE (gradient field echo) types of pulse sequences having a shorter acquisition time per one time of RF excitation. Particularly, in recent years, a segmented FFE method is frequently used. This seg. FFE method also makes use of the ECG gating technique so that acquisition timing in each segment (i.e. for each heart beat) agrees with each other.
However, the foregoing imaging techniques in which the ECG gating is utilized pose the following unsolved problems.
In other words, there is a problem of optimizing or appropriately setting cardiac synchronization timing. Concerning this problem, the truth is that precise and practical researches have hardly been made in the past. It is assumed that more proper synchronization timing values will surely be present which depend on differences in individuals of patients, differences in regions to be diagnosed (for example, whether a region is nearer to the heart or not), and types of pulse sequences used. However, practical researches or proposals about this synchronization timing have not been made yet. Because of these situations, utilizing an ECG gating technique results in that an operator determines the synchronization timing value by his or her experience or in the try and error approach. In the foregoing imaging techniques, however, a flow void phenomenon is likely to occur, and if occurring, the strength of detected echo signals is reduced, leading to unsatisfied MRA images in which desired flows of blood are not truly captured. Therefore, there is no guarantee that imaging is performed in a steady manner in which the ECG gating is fully effective and objects to be imaged are steadily captured.
For example, for imaging the aorta whose flow speed is relatively high, such as 2 m/sec, there is a possibility that a flow void phenomenon will be caused unless the cardiac synchronization timing is properly set, resulting in failure in imaging.